1. Field of the Invention
The present invention relates to the anti-oxidation protection of products in composite materials containing carbon.
2. Prior Art
The composite materials known as thermostructural materials are used for producing pieces which are required to show good thermomechanical properties, particularly in the spacial, aeronautical or braking fields. These materials are formed by a refractory fibrous reinforcement, such as in carbon or ceramics, densified by a matrix, which may also be in carbon or ceramics.
For applications implying keeping the pieces at high temperature in an oxidizing medium, the composites with a ceramic matrix may be preferred to the composites with a carbon matrix because of the poor resistance of carbon to oxidation.
Nevertheless, an anti-oxidation protection remains necessary for composites with ceramic matrix when these contain carbon. This is the case with carbon-ceramic type composites in which the fibrous reinforcement is in carbon, such as the C--SiC (carbon-silicon carbide) composites. It is also the case with ceramic-ceramic type composites, such as SiC--SiC (silicon carbide-silicon carbide) comprising an interphase layer of pyrolytic carbon between the fibers and the matrix, as described in U.S. Pat. No. 4,752,503.
The anti-oxidation protection of thermostructural composites has been the subject of much research.
One known solution consists in forming on the piece in composite material, a coating resistant to wear and to oxidation, generally a coating in silicon carbide (SiC) using the chemical vapor deposition technique.
In practice, however, it is impossible to produce such an external coating without faults appearing either during production (defects in the form of pin-holes), or when the product is used (cracks). The composite material then is not thoroughly isolated from the ambient oxidizing medium.
To eliminate these faults, it has been proposed to provide an additional coating in a material such as borate- or borosilicate-type glass which, by passing through a paste state at a certain temperature, stops the holes and cracks in the SiC coating.
Another known solution consists in providing an internal type protection by chemical vapor deposition of boron within the residual porosity of the composite material, as described in U.S. Pat. No. 4,582,751.